In a prior art dosing device, samples are arranged in sample vessels in a magazine. The sample vessels consist of sample bottles which are closed by a septum. A sample-taking unit is provided on a conveying arm which can be moved according to a program above each of the sample bottles, a rinsing vessel, or the sample inlet. The sample-taking unit comprises a dosing needle which can be introduced into a sample bottle, by penetrating the septum, into the sample inlet or into the rinsing vessel. The change-over valve comprises a stator and a rotor which can be rotated relative to the stator between a first and a second position. The rotor has the sample inlet. The rotor engages the stator with the sealing body. In the first position, the change-over valve connects the sample inlet to one end of a dosing loop and the other end of the dosing loop to a waste port. In the second position, the change-over valve connects one end of the dosing loop to a carrier liquid port and the other end of the dosing loop to an analyzing apparatus port, i.e., a port which leads to a chromatographic separation column, for example. The rear end of the dosing needle communicates through a capillary with a sample pump and a rinsing pump. The rinsing pump has check-valves and aspirates carrier liquid which can be passed by a rinsing process through the dosing needle to the rinsing vessel. Thus, the dosing needle and the capillary are filled up to the tip of the dosing needle by a carrier liquid. Further, the capillary is connected to a sample pump. The sample pump aspirates a volume of a carrier liquid. Thereby, the carrier liquid is sucked back out of the dosing needle so that another medium, air, or sample liquid is aspirated from the tip into the dosing needle.
During operation, the dosing needle is moved above the sample bottle which is located in a predetermined magazine position. First, the dosing needle penetrates only the septum of the sample bottle without dipping into the liquid. Then, the dosing needle is again moved out of the sample bottle. After returning to the starting position, a small volume of air is aspirated by the sample pump. This volume of air serves as a separating layer between the carrier liquid and the sample liquid which shall be aspirated. After a waiting period, the dosing needle penetrates the septum for a second time and now, dips into the sample liquid. The sample pump aspirates a predetermined sample volume and, in addition, a predetermined excess volume into the dosing needle and the dosing capillary. Then, the dosing needle is lifted. The conveying arm moves the dosing needle above the sample inlet of the change-over valve.
In the prior art dosing devices, first, the change-over valve is in its second position in which the dosing loop communicates with a carrier liquid port and an analyzing apparatus port. In this position a part of the excess volume is pressed into the sample inlet in order to rinse the sample inlet with sample liquid. This sample liquid is used for rinsing and runs to a waste vessel. After a waiting period, the change-over valve is rotated to the first position. The sample pump presses the predetermined sample volume into the dosing loop. The displaced volume of carrier liquid is discharged through a capillary. After a further waiting period, the change-over valve is rotated to the second position. The dosed sample volume is conveyed to the analyzing apparatus, here a chromatographic separation column, by a carrier liquid flow which is supplied through the carrier liquid port by a carrier liquid pump. Then, the sample pump pressees the sample liquid and the aspirated air into the sample inlet and through this to a waste port. Subsequently, the rinsing pump carries out a stroke cycle. Hereby, the dosing needle and the sample inlet are rinsed with carrier liquid. This carrier liquid pumped by the rinsing pump also runs through the sample inlet to the waste port. After that, the dosing needle is lifted. The conveying arm moves the dosing needle to a position of rest above the rinsing vessel.
Such arrangements are known from German Patent No. 30 30 3963, which is cognate with U.S. Pat. No. 4,413,534, granted Nov. 8, 1983, and German Patent No. 30 37 014 which is cognate with U.S. Pat. No. 4,393,726, granted Jul. 19, 1983.
Concerning prior art, in the aforementioned German Patent No. 30 30 396 valve of the present type is described which serves as a sample inlet valve and in which a dosing needle is introduced into the rotor up to the engaging surface of the stator, such that it does not penetrate into the stator beyond the engaging surface. For this purpose, the dosing bore in the stator has a smaller diameter than the dosing needle. After supplying the sample to the dosing loop, the rotor is rotated to the second position of the change-over valve. During that time, the dosing needle remains in its position in the rotor. This is necessary so that, by the piston effect of the dosing needle, sample liquid is not re-aspirated from the stator to the rotator. However, during the change-over process, the end of the dosing needle scratches along the engaging surface of the stator. This causes undesired wear and requires the use of particularly hardened engaging surfaces.
According to the aforementioned German Patent No. 30 30 396, an air volume is aspirated into the dosing needle before aspirating the sample liquid. In order to supply the sample to the dosing loop, the dosing needle is introduced with its front end through a bore of the rotor up to the dosing bore of the stator. After supplying the sample to the dosing loop, the air volume is pressed into the dosing loop. Then, before changing over the change-over valve, the dosing needle is withdrawn. Then, through the piston effect during the withdrawl of the dosing needle, only air is re-aspirated while the sample liquid entirely remains within the stator.
In the dosing arrangement according to the aforementioned German Patent No. 30 30 396, the end of the dosing needle is sealingly held with its circumference in the sealing disc of the change-over valve. This is also the case in prior art apparatus which are already in existence.
Thereby, the sealing of the dosing needle in the change-over valve is insufficient. The sealing of the dosing needle undergoes rapid wear. Because of the insufficient sealing, the guideway of the dosing needle has to be rinsed after each dosing.
Further, the volume and the diameter of the dosing loop influence the dosing. The septum of the sample bottle influences the outer moistening of the dosing needle. Finally, when dosing into the sample loop, a counterpressure is required which is generated by a flow resistance in the drain conduit.
German Patent No. 25 07 260 which is cognate with U.S. Pat. No. 4,111,051 granted Sept. 5, 1978, describes a method of supplying a sample in the flameless atomic absorption spectroscopy, in which method a sample liquid is transferred from a sample vessel to a graphite furnace for the electrothermal atomization of the sample. In the method described there, different samples are consecutively aspirated from sample vessels by means of a dosing tube. Subsequently, the dosing tube is moved to the graphite furnace, and the aspirated sample is supplied to this graphite furnace. Between each of the aspirating and supplying cycles of different samples, a rinsing process takes place. For this rinsing process, rinsing liquid is passed from the end remote from the aspirating end into the dosing tube and is supplied through this dosing tube to a rinsing vessel. After the rinsing process, the interior of the dosing tube is filled with rinsing liquid. After the rinsing process and before aspirating the sample, a small volume of air is aspirated into the dosing tube. After supplying the sample to the graphite tube, this air volume, together with a sample volume, is again pressed out of the dosing tube. In the prior art arrangement, an air and a sample pump, respectively, are provided in order to aspirate the air volume and the sample volume. Further, a rinsing liquid pump which aspirates rinsing liquid from a rinsing liquid vessel and pumps only in one direction communicates through check valves with the dosing tube. German Patent No. 36 03 632 shows a particular construction of such a rinsing liquid pump.
German Patent No. 31 25 632 which is cognate with U.S. Pat. No. 4,461,185, granted Jul. 24, 1984, shows an injection cannula for taking a liquid sample from sample vessels which are closed by a self-sealing membrane. This injection cannula consists of a capillary tube which is conically tapered at its ends, as well as at its interior.
German Published Patent Application No. 37 20 289 describes a method of electrothermal atomization of samples in which the sample is pressed by a carrier liquid through a dosing tube through a heated capillary, with the sample precipitating at the walls of a pre-heated graphite furnace. Then, the thus-precipitated sample is detected by the atomic absorption spectroscopy.